This application claims priority to Japanese Patent Application No. 2000-357858, filed on Nov. 24, 2000.
The present invention relates to a golf club shaft made of fiber-reinforced plastics (FRP), and in particular to a FRP golf club shaft which facilitates a swing by significantly suppressing deformation of the shaft cross section, a golf club having such a shaft, and a novel method for evaluating golf club shafts for deflection.
A golf club shaft made of fiber-reinforced plastics (hereinafter referred to as an xe2x80x98FRP golf club shaftxe2x80x99) is advantageous over a metal golf club shaft as it is lighter than metal one and is easier to accelerate a swing, thereby increasing the flying distance of a ball. Thus, the FRP golf club shaft is extensively employed.
The FRP shaft is a shaft formed of reinforcing fibers impregnated with resin. The FRP shaft includes a shaft fabricated in the sheet-rolling process (SIR shaft), a shaft fabricated in the filament-winding process (FW shaft), and a braided shaft. The S/R shaft is formed by winding unidirectional prepreg sheets made of reinforcing fibers over a mandrel. The FW shaft is formed by winding fiber bundles of reinforcing fibers (yarns) over a mandrel while reciprocating them along the longitudinal axis of the mandrel. The braided shaft is formed by braiding a plurality of fiber bundles of reinforcing fibers (yarns) or tow prepregs (or yarn prepregs) while braiding them over the mandrel to the substantially entire length of the shaft. In manufacturing any of the shafts, the reinforcing fibers may be impregnated with resin before or after winding the fibers around the mandrel.
The conventional FRP shafts, however, suffer from drawbacks in terms of deflection of the shafts during a swing as described below.
Consider the state of the shaft during a swing. The golfer causes generally rotational motion of the head of the club during a swing to hit a ball. During the swing, some possible forces are applied to the shaft, that is, (1) centrifugal force immediately before the impact, (2) inertial force caused by acceleration or deceleration of the head, and (3) impact force immediately after the impact. More specifically, the centrifugal force of (1) is 300 to 500 N which is generated immediately before the impact, when the head speed reaches 40 to 50 m/s. This force pulls the entire shaft in the centrifugal direction of the rotational motion and causes the shaft the bending deformation and tensile deformation. The inertial force of (2) originates in acceleration or deceleration of the head when the golfer rotates, twists, or translates his waist, arms, or wrists. This force applies bending or torsional moment on the shaft, thus causing its bending or torsional deformation. The impact force of (3) is conveyed from the head to the shaft immediately after the impact. This force causes various deformations of the shaft depending on the hitting point of the ball.
An analysis of the force applied to the shaft before the impact, such as the forces (1) and (2), shows that the force may be divided into (A) tensile stress and compression stress symmetrical to the neutral plane that are caused by bonding moment load on the shaft that is applied in the direction of the shaft, (B) tensile stress in the longitudinal direction that is caused by centrifugal force, and (C) shearing force caused by microscopic torsional load, which is negligibly small. The neutral plane of (A) means a virtual plane located along the longitudinal axis of the shaft upon which no tensile stress and compression stress act. Therefore, at a given position in the longitudinal direction of the shaft, deformations are created in the shaft""s longitudinal direction and in the shaft""s circumferential direction due to tensile stress or compression stress. Then the shaft cross section, which generally assumes a circular shape before the swing, deforms elliptically due to collapse or flattening, which sometimes affects the swing or the feeling of the club when the golfer swings it. Furthermore, since the degree of shaft""s deflection is greater in case of golfers who are power hitters or who swing faster, the deflection is especially a serious problem for male professional golfers.
JP-A-11-33151 disclosed an S/R shaft, which materializes light weight and high elasticity and which prevents decreases in strength. Decreases in strength of shaft are prevented by having a high elasticity layer of circumferential fibers in which the fibers are directed in the circumference direction relative to the shaft axis. However, since the thickness of the layer of the circumferential fibers is 0.023 mm and this thickness is relatively thinner compared to that of the entire shaft, resistance against deformation of the layer, or contribution to shaft rigidity, is low. In addition, since this is the S/R shaft, junctions exist at the start of the winding of each reinforcing-fiber sheet and at a location between different sheets (or inter-layer location), which is undesirable in terms of the strength of the shaft.
JP-A-2000-14843 disclosed an S/R shaft using triaxial fabric layers as the reinforcing-fiber layers for providing different required characteristics, such as flexural rigidity, torsional rigidity, and anti-collapse rigidity along the longitudinal direction of the shaft with this shaft, the required characteristics may be satisfied by varying, along the axial direction of the shaft, at least one of following: the fiber density, type of fibers, and the physical properties of tri-directional yarns that constitute the triaxial fabric layers. In fabricating this shaft, triaxial fabric layers are partly overlapped and discontinuously wound over the mandrel along the longitudinal direction of the shaft. This makes the shaft undesirable in terms of the strength.
Conventionally, no index has been established for objectively evaluating the degree of shaft deflection, and the manufacturing of shafts has largely depended on manufacturer""s experience.
The object of the present invention is to provide an FRP golf club shaft that facilitates swing by evaluating shaft deflection caused by the above-mentioned load applied to the shaft on swinging or before the impact, based on Poisson""s ratio, and by suppressing deformation of the shaft cross section as best as possible.
A golf club shaft made of fiber reinforced plastic comprising at least a portion along the longitudinal direction of the shaft which satisfies a Poisson""s ratio of 0.5 or less The Poisson""s ratio is expressed by the ratio of lateral strain to longitudinal strain. The longitudinal strain is the strain in the longitudinal direction of the shaft and the lateral strain is the strain in the circumferential direction of the shaft when load is applied to the shaft.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.